Receiving circuit for optical communication

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-61191, filed on Mar. 3, 2009; the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to a receiving circuit for optical communication, and, more particularly, is suitably applied to a receiving circuit for optical communication used in complementary optical transmission.

2. Description of the Related Art

In optical communication, photoelectric conversion is performed by using a light receiving element. When the pulse width of signal light is long, current consumption increases because an electric current flows to the light receiving element for a long time. Therefore, there is a method of using a complementary optical wiring circuit to prevent the electric current from flowing to the light receiving element for a long time. The complementary optical wiring circuit transmits only a rising edge (set pulse light) and a falling edge (reset pulse light) of the signal light and restores an original signal from the rising edge and the falling edge.

For example, Japanese Patent Application Laid-Open No. 2001-285195 discloses a method for suppressing output drift of a complementary optical wiring circuit. The complementary optical wiring circuit includes first and second diode-type light emitting elements serially connected to each other, a coupling capacitor connected to a connection point of the first and second diode-type light emitting elements, a pulse signal source for alternately applying a differential waveform signal to the first and second diode-type light emitting elements via the coupling capacitor, and a DC bias voltage source connected between an anode of the first diode-type light emitting element and a cathode of the second diode-type light emitting element.

However, the receiving circuit for optical communication in the past performs photoelectric conversion for the set pulse light and photoelectric conversion for the reset pulse light independently from each other. Therefore, when noise rides on the set pulse light or the reset pulse light during the transmission of the set pulse light and the reset pulse light, the noise cannot be eliminated.

A receiving circuit for optical communication according to an embodiment of the present invention comprises: a first photodiode that receives a set pulse corresponding to a rising edge of signal light; a second photodiode that receives a reset pulse corresponding to a falling edge of the signal light; a differential circuit, one differential input terminal of which is connected to an anode side of the first photodiode and the other differential input terminal of which is connected to an anode side of the second photodiode; a latch circuit that performs latch operation based on output of the differential circuit; and a flip-flop that performs set operation and reset operation based on a latch state of the latch circuit.

FIG. 1 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a first embodiment of the present invention;

FIG. 2 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 1 ;

FIG. 3 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a second embodiment of the present invention;

FIG. 4 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 3 ;

FIG. 5 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a third embodiment of the present invention; and

FIG. 6 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 5 .

Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

FIG. 1 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a first embodiment of the present invention.

In FIG. 1 , the receiving circuit for optical communication includes photodiodes D 1 and D 2 . The photodiode D 1 can receive set pulse light OPTt corresponding to a rising edge of signal light. The photodiode D 2 can receive reset pulse light OPTc corresponding to a falling edge of the signal light.

The receiving circuit for optical communication also includes a differential circuit 11 to which the outputs from the photodiodes D 1 and D 2 are given as differential inputs, a latch circuit 12 that performs latch operation based on the output of the differential circuit 11 , a flip-flop SR 1 that performs set operation and reset operation based on a latch state of the latch circuit 12 , a pre-charge circuit 13 that shifts, based on the latch state of the latch circuit 12 , the potentials of both differential input terminals of the differential circuit 11 to a state in which the signal light is not input.

The differential circuit 11 includes N-channel field effect transistors (hereinafter, “Nch transistors”) N 0 to N 2 . A drain of the Nch transistor N 0 is connected to sources of the Nch transistors N 1 and N 2 and a source thereof is grounded. A gate of the Nch transistor N 1 is connected to an anode of the photodiode D 1 and connected to a capacitor C 1 . A gate of the Nch transistor N 2 is connected to an anode of the photodiode D 2 and connected to a capacitor C 2 .

The latch circuit 12 includes Nch transistors N 3 and N 4 and P-channel field effect transistors (hereinafter, “Pch transistors”) P 1 and P 2 . The Nch transistor N 3 and the Pch transistor P 1 configure an inverter with gates and drains thereof connected in common to each other. The Nch transistor N 4 and the Pch transistor P 2 configure an inverter with gates and drains thereof connected in common to each other. The pair of inverters are cross-coupled with an input of one inverter connected to an output of the other inverter and an output of one inverter connected to an input of the other inverter to form storage nodes Nt and Nc of the latch circuit 12 .

The drain of the Nch transistor N 3 , the drain of the Pch transistor P 1 , the gate of the Nch transistor N 4 , and the gate of the Pch transistor P 2 are connected to a set terminal of the flip-flop SR 1 . The drain of the Nch transistor N 4 , the drain of the Pch transistor P 2 , the gate of the Nch transistor N 3 , and the gate of the Pch transistor P 1 are connected to a reset terminal of the flip-flop SR 1 .

The drain of the Nch transistor N 3 and the drain of the Pch transistor P 1 are connected to the drain of the Pch transistor P 3 . The drain of the Nch transistor N 4 and the drain of the Pch transistor P 2 are connected to the drain of the Pch transistor P 4 .

The pre-charge circuit 13 includes Nch transistors N 5 and N 6 , an exclusive OR circuit XR 1 , and an inverter IV 1 . A drain of the Nch transistor N 5 is connected to the gate of the Nch transistor N 1 . A drain of the Nch transistor N 6 is connected to the gate of the Nch transistor N 2 .

The drains of the Nch transistor N 3 and the Pch transistor P 1 and the gates of the Nch transistor N 4 and the Pch transistor P 2 are connected to one input terminal of the exclusive OR circuit XR 1 . The drain of the Nch transistor N 4 , the drain of the Pch transistor P 2 , the gate of the Nch transistor N 3 , and the gate of the Pch transistor P 1 are connected to the other input terminal of the exclusive OR circuit XR 1 .

An output terminal of the exclusive OR circuit XR 1 is connected to gates of the Nch transistors N 5 and N 6 . The output terminal of the exclusive OR circuit XR 1 is connected to gates of the Nch transistor N 0 and the Pch transistors P 3 and P 4 via the inverter IV 1 .

FIG. 2 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 1 .

In FIG. 2 , it is assumed that input voltages Xt and Xc to the differential circuit 11 is at a low level and output voltages Vtn and Vcn from the latch circuit 12 are at a high level. Output voltage PCp is at the low level and output voltage PCn is at the high level.

The Nch transistors N 5 and N 6 and the Pch transistors P 3 and P 4 are turned off and the Nch transistor N 0 is turned on. As a result, the differential circuit 11 is maintained in a state in which differential operation is possible and the latch circuit 12 is maintained in a state in which latch operation is possible.

When the set pulse light OPTt is made incident on the photodiode D 1 (t 1 ), the set pulse light OPTt is photoelectrically converted. The input voltage Xt changes from the low level to the high level (t 2 ). When the gate potential of the Nch transistor N 1 changes to the high level, the Nch transistor N 1 is turned on. When the output voltage Vcn is at the high level, the Nch transistor N 3 is turned on. Therefore, the output voltage Vtn changes to the low level (t 3 ).

When the output voltage Vtn changes to the low level, output voltage out changes from the low level to the high level (t 4 ). The output voltage PCp changes from the low level to the high level (t 5 ). Then, the Nch transistors N 5 and N 6 are turned on. The input voltage Xt changes from the high level to the low level (t 6 ). The input voltage Xc is maintained at the low level.

When the output voltage PCp changes from the low level to the high level, the output voltage PCn changes from the high level to the low level (t 7 ). Then, the Nch transistor N 0 is turned off and the Pch transistors P 3 and P 4 are turned on. The output voltage Vtn changes from the low level to the high level (t 8 ). The output voltage Vcn is maintained at the high level.

When the output voltage Vtn changes to the high level, the output voltage PCp changes from the high level to the low level (t 9 ). Then, the Nch transistors N 5 and N 6 are turned off. The output voltage PCn changes from the low level to the high level (t 10 ). The Nch transistor N 0 is turned on and the Pch transistors P 3 and P 4 are turned off.

When the reset pulse light OPTc is made incident on the photodiode D 2 (t 11 ), the reset pulse light Optic is photoelectrically converted. The input voltage Xc changes from the low level to the high level (t 12 ). The gate potential of the Nch transistor N 2 is at the high level and the Nch transistor N 2 is turned on. When the output voltage Vtn is at the high level, the Nch transistor N 4 is turned on. Therefore, the output voltage Vcn changes to the low level (t 13 ).

When the output voltage Vcn changes to the low level, the output voltage out changes from the high level to the low level (t 14 ). The original signal light is restored. The output voltage PCp changes from the low level to the high level (t 15 ). Then, the Nch transistors N 5 and N 6 are turned on. The input voltage Xc changes from the high level to the low level (t 16 ). The input voltage Xt is maintained at the low level.

When the output voltage PCp changes from the low level to the high level, the output voltage PCn changes from the high level to the low level (t 17 ). Then, the Nch transistor N 0 is turned off and the Pch transistors P 3 and P 4 are turned on. The output voltage Vcn changes from the low level to the high level (t 18 ). The output voltage Vtn is maintained at the high level.

When the output voltage Vcn changes to the high level, the output voltage PCp changes from the high level to the low level (t 19 ). Then, the Nch transistors N 5 and N 6 are turned off. The output voltage PCn changes from the low level to the high level (t 20 ). The Nch transistor N 0 is turned on and the Pch transistors P 3 and P 4 are turned off.

Consequently, when the set pulse light OPTt and the reset pulse light Optic are not input, it is possible to interrupt all current paths of the receiving circuit for optical communication and realize a reduction is power consumption.

The outputs from the photodiodes D 1 and D 2 are given to the differential circuit 11 as the differential inputs. Therefore, it is possible to eliminate common mode noise superimposed on the set pulse light OPTt and the reset pulse light OPTc and improve noise resistance in complementary optical transmission.

FIG. 3 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a second embodiment of the present invention.

In FIG. 3 , the receiving circuit for optical communication includes a pre-charge circuit 13 ′ instead of the pre-charge circuit 13 shown in FIG. 1 . The pre-charge circuit 13 ′ includes Nch transistors N 5 to N 8 and inverters IV 2 and IV 3 . The Nch transistor N 7 is connected in series to the Nch transistor N 5 . Nch transistor N 8 is connected in series to the Nch transistor N 6 .

The drain of the Nch transistor N 3 , the drain of the Pch transistor P 1 , the gate of the Nch transistor N 4 , and the gate of the Pch transistor P 2 are connected to a gate of the Nch transistor N 5 via the inverter IV 2 . The drain of the Nch transistor N 3 , the drain of the Pch transistor P 1 , the gate of the Nch transistor N 4 , and the gate of the Pch transistor P 2 are connected to a gate of the Nch transistor N 8 .

The drain of the Nch transistor N 4 , the drain of the Pch transistor P 2 , the gate of the Nch transistor N 3 , and the gate of the Pch transistor P 1 are connected to a gate of the Nch transistor N 6 via the inverter IV 3 . The drain of the Nch transistor N 4 , the drain of the Pch transistor P 2 , the gate of the Nch transistor N 3 , and the gate of the Pch transistor P 1 are connected to a gate of the Nch transistor N 7 .

FIG. 4 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 3 .

In FIG. 4 , it is assumed that the input voltage Xt and Xc to the differential circuit 11 are at a low level and the output voltages Vtn and Vcn from the latch circuit 12 are at a high level. The output voltage PCp is at the low level and the output voltage PCn is at the high level.

The Pch transistors P 3 and P 4 are turned off and the Nch transistors N 0 , N 7 , and N 8 are turned on. The Nch transistors N 5 and N 6 are turned off.

As a result, the differential circuit 11 is maintained in a state in which differential operation is possible. The latch circuit 12 is maintained in a state in which latch operation is possible.

When the set pulse light OPTt is made incident on the photodiode D 1 (t 31 ), the set pulse light OPTt is photoelectrically converted. The input voltage Xt changes from the low level to the high level (t 32 ). When the gate potential of the Nch transistor N 1 changes to the high level, the Nch transistor N 1 is turned on. When the output voltage Vcn is at the high level, the Nch transistor N 3 is turned on. Therefore, the output voltage Vtn changes to the low level (t 33 ).

When the output voltage Vtn changes to the low level, the output voltage out changes from the low level to the high level (t 34 ). The output voltage PCp changes from the low level to the high level (t 35 ). Then, the output voltage PCn changes from the high level to the low level (t 36 ).

When the output voltage Vtn changes to the low level, the Nch transistor N 5 is turned on. The input voltage Xt changes from the high level to the low level (t 37 ). As a result, the Nch transistor N 1 is turned off in a state in which the Nch transistors N 5 and N 7 are on. The capacitor C 1 is pre-charged.

When the output voltage PCn changes from the high level to the low level, the Nch transistor N 0 is turned off and the Pch transistors P 3 and P 4 are turned on. The output voltage Vtn changes from the low level to the high level (t 38 ). The output voltage Vcn is maintained at the high level.

When the output voltage Vtn changes to the high level, the output voltage PCp changes from the high level to the low level (t 39 ). Then, the output voltage PCn changes from the low level to the high level (t 40 ). The Nch transistor N 0 is turned on and the Pch transistors P 3 and P 4 are turned off.

When the reset pulse light OPTc is made incident on the photodiode D 2 (t 41 ), the reset pulse light Optic is photoelectrically converted. The input voltage Xc changes from the low level to the high level (t 42 ). The gate potential of the Nch transistor N 2 changes to the high level and the Nch transistor N 2 is turned on. When the output voltage Vtn is at the high level, the Nch transistor N 4 is turned on. Therefore, the output voltage Vcn changes to the low level (t 43 ).

When the output voltage Vcn changes to the low level, the output voltage out changes from the high level to the low level (t 44 ). The original signal light is restored. The output voltage PCp changes from the low level to the high level (t 45 ). Then, the output voltage PCn changes from the high level to the low level (t 46 ).

When the output voltage Vcn changes to the low level, the Nch transistor N 6 is turned on. The input voltage Xc changes from the high level to the low level (t 47 ). As a result, the Nch transistor N 2 is turned off in a state in which the Nch transistors N 6 and N 8 are on. The capacitor C 2 is pre-charged.

When the output voltage PCn changes from the high level to the low level, the Nch transistor N 0 is turned off and the Pch transistors P 3 and P 4 are turned on. The output voltage Vcn changes from the low level to the high level (t 48 ). The output voltage Vtn is maintained at the high level.

When the output voltage Vcn changes to the high level, the output voltage PCp changes from the high level to the low level (t 49 ). Then, the output voltage PCn changes from the low level to the high level (t 50 ). The Nch transistor N 0 is turned on and the Pch transistors P 3 and P 4 are turned off.

Consequently, immediately after the set pulse light OPTt is made incident on the photodiode D 1 , only the capacitor C 1 can be pre-charged. The capacitor C 2 can be prevented from being pre-charged. Therefore, even when the reset pulse light Optic is made incident on the photodiode D 2 , the reset pulse light Optic can be detected by the differential circuit 11 . It is possible to realize a reduction in power consumption while preventing operation speed from being limited by the pre-charge.

FIG. 5 is a block diagram of a schematic configuration of a receiving circuit for optical communication according to a third embodiment of the present invention.

In FIG. 5 , the receiving circuit for optical communication includes resistors R 1 and R 2 in addition to the components of the receiving circuit for optical communication shown in FIG. 1 . The resistor R 1 is connected to the gate of the Nch transistor N 1 and the resistor R 2 is connected to the gate of the Nch transistor N 2 .

FIG. 6 is a timing chart of the operation of the receiving circuit for optical communication shown in FIG. 5 .

In FIG. 6 , it is assumed that the input voltages Xt and Xc to the differential circuit 11 are at a low level and the output voltages Vtn and Vcn from the latch circuit 12 are at a high level. The output voltage PCp is at the low level and the output voltage PCn from the inverter IV 1 is at the high level.

The Nch transistors N 5 and N 6 and the Pch transistors P 3 and P 4 are turned off and the Nch transistor N 0 is turned on.

When the resistors R 1 and R 2 are not provided, if leakage due to dark currents occurs in the photodiodes D 1 and D 2 , the potentials of the input voltages Xt and Xc gradually rise. When the input voltage Xt reaches threshold voltage of the Nch transistor N 2 (t 62 ), the Nch transistor N 2 is turned on. When the output voltage Vtn is at the high level, the Nch transistor N 4 is turned on. Therefore, the output voltage Vcn changes to the low level (t 63 ).

When the output voltage Vcn changes to the low level, the output voltage out changes from the high level to the low level (t 64 ). The original signal light is restored. The output voltage PCp changes from the low level to the high level (t 65 ). Then, the Nch transistors N 5 and N 6 are turned on. The input voltage Xc changes from the high level to the low level (t 66 ). The input voltage Xt is maintained at the low level.

When the output voltage PCp changes from the low level to the high level, the output voltage PCn changes from the high level to the low level (t 67 ). Then, the Nch transistor N 0 is turned off and the Pch transistors P 3 and P 4 are turned on. The output voltage Vcn changes from the low level to the high level (t 68 ). The output voltage Vtn is maintained at the high level.

When the output voltage Vtn changes to the high level, the output voltage PCp changes from the high level to the low level (t 69 ). Then, the Nch transistors N 5 and N 6 are turned off. The output voltage PCn changes from the low level to the high level (t 70 ). The Nch transistor N 0 is turned on and the Pch transistors P 3 and P 4 are turned off.

As a result, regardless of the fact that the reset pulse light Optic is not made incident on the photodiode D 2 , the output voltage out changes from the high level to the low level to cause malfunction.

On the other hand, when the resistors R 1 and R 2 are provided, dark currents in the photodiodes D 1 and D 2 are caused to escape via the resistors R 1 and R 2 , respectively. Therefore, even when leakage due to the dark currents occurs in the photodiodes D 1 and D 2 , it is possible to prevent the potentials of the input voltages Xt and Xc from gradually rising and prevent malfunction of the receiving circuit for optical communication.

In the receiving circuit for optical communication according to the first embodiment shown in FIG. 1 , the pre-charge circuit 13 and the Pch transistors P 3 and P 4 are provided. However, the pre-charge circuit 13 and the Pch transistors P 3 and P 4 do not have to be provided.

In the receiving circuit for optical communication according to the third embodiment shown in FIG. 5 , the resistors R 1 and R 2 are provided in addition to the components of the receiving circuit for optical communication according to the first embodiment shown in FIG. 1 . However, the resistors R 1 and R 2 can be provided in addition to the components of the receiving circuit for optical communication according to the second embodiment shown in FIG. 3 . Alternatively, the resistors R 1 and R 2 can be provided in the receiving circuit for optical communication according to the first embodiment shown in FIG. 1 from which the pre-charge circuit 13 and the Pch transistors P 3 and P 4 are removed.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.